At the present time, photographic processes using silver halide have been most widely used because of their excellent photographic properties such as sensitivity or control of gradation, etc., as compared with other photographic processes, such as an electrophotographic process or a diazo photographic process. However, recently many techniques have been developed which are capable of easily and quickly obtaining images by changing the conventional wet process using a developing solution into a dry process such as a process using heat.
Heat-developable photographic materials are known in the field of these techniques, and heat-developable photographic materials and processes have been described in U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, No. 17029, pages 9 to 15 (June, 1978).
Many different processes for obtaining color images have been proposed. With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with a couplers, it has been proposed to use a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as described in U.S. Pat. No. 3,531,286, a p-aminophenol type reducing agent as described in U.S. Pat. No. 3,761,270, a sulfonamidophenol type reducing agent as described in Belgian Patent No. 802,519 and the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler as described in U.S. Pat. No. 4,021,240.
Another process comprises introducing a nitrogen containing heterocyclic group into a dye, forming a silver salt and releasing a dye by heat development. This process is described in Research Disclosure, No. 16966, pages 54 to 58 (May, 1978).
Also, processes for forming a positive color image by a heat developable dye bleach process, with useful dyes and methods for bleaching have been described, for example, in Research Disclosure, No. 14433, pages 30 to 32 (April, 1976), ibid., No. 15227, pages 14 to 15 (December, 1976) and U.S. Pat. No. 4,235,957.
Moreover, a process for forming a color image utilizing a leuco dye has been described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617.
However, these known processes have a serious drawback, that is, the gradual formation of color fog. The fog occurs because of the oxidation reduction reaction or color forming reaction, etc., which gradually occurs during storage of the photographic material. The fog forming reactions occur due to the presence of a silver halide, an organic silver salt oxidizing agent, a color image forming substance and a reducing agent all in contact with each other.
The above mentioned processes are also undesirable because discoloration and/or fading of the color image or coloration of the white background portion, etc., occurs because developed silver, a color image and the remaining developing agent are coexistent after the heat development procedure. To eliminate this disadvantage, there has been proposed a process which comprises transferring the resulting color image by heat to an image receiving sheet in the presence of a solvent such as methanol, etc., as described in U.S. Pat. Nos. 3,531,286, 3,761,270 and 4,021,240, etc. However, when obtaining a multicolor image, it is necessary, for example, to conduct the heat transfer from a multilayer heat-developable color photographic element comprising at least a yellow color image forming layer, a magenta color image forming layer and a cyan color image forming layer. Accordingly, the process tends to hardly transfer the dye from the color image forming layer which is located far from the image receiving sheet. This results in destroying the color balance which is a serious defect.
Furthermore, when preparing a coating solution of these heat-developable color photographic materials, various components such as a silver halide, an organic silver salt oxidizing agent, a reducing agent and a coloring material, for example, a coupler, a leuco dye, etc., and if desired, a base and a thermal solvent are coexistent and maintained at a temperature from 40.degree. C. to 50.degree. C. for a certain period. While maintaining these materials in solution, an oxidation reduction reaction or color forming reaction occurs resulting in the degradation of image quality. For example, coloration of the white background portion of the heat-developable color photographic material may occur, which is also a problem.